Zirconium boride/diboride is emerging as a potential advanced ceramic because of it's excellent properties—high melting point, hardness, elastic modulus and electrical conductivity, resistance to acids like HCL, HF and other non-ferrous metals, cryolite and non-basic slags. Due to these properties, ZrB2 finds several engineering applications like cathodes for electrochemical processing of aluminum (Hall-Heroult process), evaporation boats, crucibles for handling molten metals, thermowells, wear parts, nozzles, armor and as dispersoid in metal and ceramic composites for getting improved mechanical properties, cutting tools etc. It is also used as thermocouple sieves for high temperature furnaces.
Zirconium Boride due to its several technological important uses, as stated above, has been synthesized in the prior art in several ways:    1. Synthesis from elements by melting, sintering or hot pressing in the process.
Zirconium and boron metal ingots are melted together in inert atmosphere in furnace to obtain a final product in the form of lumps of zirconium boride. Metallurgical processes such as forging, milling are used to obtain fine powder of zirconium boride. The process uses raw material in elemental form. Therefore different powder metallurgy processing methods are required for converting from elemental to powder form. Thus the process becomes very costly and so commercially may not be viable.    2. Borothermic reduction of metal oxides
In this method, zirconium oxide is reduced using boron metal powder in furnace under inert atmosphere, represented by the following equation.ZrO2+2B=ZrB2+O2
The use of pure boron in the process makes it costly. Another disadvantage is that the efficiency of conversion is generally not very high.    3. Another known process uses carbothermic reduction of metal-oxides and boric oxide to produce zirconium boride powder and can be represented by the following equationZrO2+B2O3+5C=ZrB2+5CO    4. In another know process, reduction of the metal oxide is done with carbon or Boron carbide, the reaction being represented by the equation.2ZrO2+B4C+3C=2ZrB2+4CO
The above mentioned processes 3 and 4, generally do not result to pure ZrB2 powder. Also reduction of ZrO2 by boron carbide and carbon requires very high temperature furnaces in the range of about 2000 to 2200° C., which makes the process costly and much more time consuming.    5. Another known process is aluminothermic, magnetiothermic and ilicothermic reduction of metal oxide—Boric Oxide mixture to produce zirconium boride powder.
In this process, mixture of oxides of zirconium and boron is coeduced using low melting metal powders of aluminum, magnesium or silicon in furnace, which is represented by the following equationZrO2+B2O3+5Mg=ZrB2+5MgO
Here though the reduction is done at relatively low temperature, but for high purity products further treatment at higher temperature is required. Also since zirconium oxide is a very stable oxide due to its low free energy it is difficult to reduce it completely without going to high temperature and hence the final product usually retains some amount of ZrO2 with zirconium boride and magnesium oxide or other metal oxide. It is difficult to remove ZrO2 with etchants because etchants which can dissolve ZrO2 also dissolve zirconium boride. Hence zirconium boride also pass into the solution.    6. Self propagating high temperature synthesis (SHS) synthesis of Zirconium boride by elemental powder.
The SHS process is in exploitation of a highly exothermic and usually very rapid chemical reaction to form an useful material. The central feature of the process is that the heat required to drive the chemical reaction is supplied from the reaction itself. The potential commercial attractiveness of the SHS derives from the expected lower capital and operating costs. The SHS has found applications in recent year for preparing intermetallics and advanced high temperature materials such as carbides, borides, slicides and nitrides (A. G. Mershanov and I. P. borovinskaya, Combat. Sci. Technol. 10, 195 (1975), I. M. Sheppard, Adv. Mater. Proce, 25, (1986). Applications, advantages, fundamental and technological aspects of SHS have been reviewed in literature [Z. A. Munir, Meatall, Trans. A, 23A, 7 (1992), A Makino, C. K. Low, J. Amer, Corm. Soc. 77(3), 778 (1994). This technique has inherent advantages over conventional methods, which require high temperature furnaces and longer processing times. Materials produced by the SHS method have advantages such as high purity of product [B. Manaly, J. P. Holt and Z. A. Munir, mat. Sci. Res., 16. 303 (1984), low energy requirements and relatives simplicity of the process (H. C. Yi and J. J. Moore, J. Mat, Sci., 25, 1150 (1990)]. Owing to the high cooling rate, high defect concentrations and non-equilibrium structures exist in the SHS produced materials, resulting in more reactive metastable and thus more sinterable products [O. R. Bermann and J. Barrington, J. Amer. Cerm. Soc., 49, 502 (1966).
In the SHS of zirconium boride, zirconium and boron metal powders are mixed together and ignited from top. The ignition source is switched off as the surface reaches the required ignition temperature. The combustion wave now propagates throughout the sample. Reaction rates has been calculated as 25 centimeter per second as reported in literature.
Even though the SHS process has advantages but use of element powder makes the process cost incentive.
Hence it is observed that in all the above known processes, the time requirement is more and also it requires high temperature furnace in the range of 2000 to 2200° C. to achieve 99% and above pure products or it does require pure elemental powders as starting raw material which make the process costly.